The present invention concerns communication systems in which messages are sent across a switching medium. The invention is particularly applicable to sending digital data by Time Division Multiplex (TDM) and to use in the Integrated Services Digital Networks (ISDN) currently being introduced. It also has applications in message switches including packet switches, ATM switches and frame relaying switches.
In an ISDN system, as in many other systems, incoming messages to a switching medium have to be translated into outgoing messages. The present invention is concerned with systems in which this translation is carried out with reference to a mapping function, that is the incoming message is used to access an address of a stored map which gives the necessary route across the switching medium. The incoming message may include a portion which identifies its intended destination and which along with its entry point in the system be used to access the local map. It is however possible merely for the entry port at which the incoming message appears to be the determining factor in assessing the local map.
Prior systems utilizing local maps for message routing across a switching medium have all had the considerable disadvantage that fault detection is extremely difficult.